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Apply an electrical charge to a cell in a memory chip and you change its state from 0 to 1or 1 to 0. Remove power from a typical memory chip and it forgets the 0s and 1s. Not so with flash memory: It maintains the state of each cell when the chip powers down. There are two downsides: Flash memory cards cost more, and they are slower than traditional hard drives. Generally speaking, the transfer rate of flash memory is about 1 MBps. New technologies and interfaces can double or triple that rate, but you will pay extra for the convenience. Either way, the slowest, cheapest DRAM chip used in your PC is vastly quicker than the fastest flash memory chip available. That's why small hard drives live on.

The market for flash memory cards evolved with the advent of digital cameras in the mid-1990s. At the time, Sony Mavica cameras, which have floppy disk drives in their back panels, were vastly preferred over those with proprietary serial cables. But once cameras surpassed 1 megapixel, the need for smaller, high-capacity, removable memory became clear.

SmartMedia (SM). The first standards-based memory cards to appear, SmartMedia units are about as thick as playing cards. But the deck is stacked against the SmartMedia card, because it has the biggest footprint (about 2.7 square inches, compared with Secure Digital's 1.2 square inches), and its flexibility makes it vulnerable to damage. It has no controller to let your PC treat it as a standard hard drive, so you must pay extra for a PC Card adapter (about $25). Also, some devices won't work with cards larger than about 16MB or 32MB.

SmartMedia's design limits it to 128MB. Fuji and Olympus used to be the biggest backers of SmartMedia, but now they are making the transition to their own format called xD-Picture Card. Expect to see few if any new devices using SmartMedia.

MultiMediaCard (MMC). Along with SmartMedia, MMC was the other early memory standard. It's less damage-prone because of its rigid plastic shell. It has the same footprint and pin-out as its successor, the Secure Digital (SD) card. MMCs fit in SD devices but not the other way around (because SD cards are thicker). SD music players typically won't play audio from an MMC, because SD players require encrypted music. Most companies are currently phasing out MMC devices. MMC loyalists are backing a format called RS MMC, which is essentially miniSD (see below) but half as thick; its fate is uncertain.

CompactFlash (CF). CF is the aging but far-from-dead flash memory champion. More devices currently use CF than any other media type. Its future likely lies in professional digital cameras that need massive capacity4GB CF cards are becoming availableand whose users don't mind the size of CF cards (roughly half of a PC Card). Makers of consumer and advanced amateur camerasled by Kodakare moving to smaller SD cards, but pros will stick with CF. Most CF cards are Type II, which are as thick as PC Cards.

PC Card memory. Type II (standard-thickness) PC Cards had some popularity among users who wanted more capacity than CF cards supply. But these cards were never used in cameras or audio devices, and now that CF and USB keys have adequate capacity, there's not much call for PC Card flash memory outside of specialized markets. The highest available capacity is 2GB (for $1,000 to $2,000); CF cards are already there and headed for 4GB.

Memory Stick. Sony created and supports the Memory Stick format across its vast product line, and several other vendors offer Memory Stick products as well. Memory Stick was introduced in 1999 and quickly grew to constitute about a quarter of the flash memory market. The standard has resulted in a confusing array of devices all called Memory Stick.

First, there is Memory Stick, which is limited to 128MB. It comes in regular and OpenMG (Magic Gate) versions; only the latter can handle audio files. A tinier version, Memory Stick Duo, is currently used in cell phones, and there are plans to include compatibility with digital cameras and music players. The Memory Stick Duo is only a bit smaller than an SD card.

To address the limited capacity of Memory Stick, Sony offers a bank-switched Memory Stick with Memory Select, two banks of 128MB you access by flipping a switch. To reach beyond 128MB of contiguous memory, Sony offers Memory Stick Pronow at 1GBbut it's generally not backward-compatible, even though it physically fits in the Memory Stick slots of older devices. (Every kind of Memory Stick works in a Memory Stick Pro slot, directly or with an adapter.)

If you live in an all-Sony world, the convenience is outstanding: You can off-load video clips and stills from a Sony DV camcorder and show them on a Sony TV or carry them on a Sony Clié handheld.

Secure Digital (SD). Secure Digital has the broadest support and brightest future. The cards are very small, however, so they're easy to lose. For audio buffs, the biggest drawback is that SD players, like Memory Stick players, require you to check out music from your PC, and the conversion process can be slow.

SD is second only to CF in capacity, with projections of 16GB by the end of 2005. A number of notebooks have added dedicated SD slots as well. An even smaller miniSD card is also available, primarily for use in cell phones, with cameras and music players coming later. Stick it in an adapter and it fits in any SD slot.

xD-Picture Card. Camera makers Fuji and Olympus have gone from the bulkiest memory card standard, SmartMedia, to the smallest, xD. xD-Picture Cards do not have controllers, which adds to the cost of a PC Card adapter, but they offer faster read and write times. xD is unlikely to surpass SD sales or to have the broad cross-device support of SD or Memory Stick, but Olympus is a major player in digital photography. If your interests lie only with digital photography, there's no downside to xD other than capacities currently lower than CF and SD.

Bill Howard is the editor of TechnoRide.com, the car site for tech fans, and writes a column on car technology for PC Magazine each issue. He is also a contributing editor of PC Magazine.
Bill's articles on PCs, notebooks, and printers have been cited five times in the annual Computer Press Association Awards. He was named as one of the industry's ten most influential journalists from 1997 to 2000 by Marketing Computers and is a frequent commentator on TV news and business shows as well...
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